Optical waveguides are incorporated in optical waveguide devices, optical integrated circuits, and optical wiring boards and are in extensive use in optical communication, optical information processing, and other general optical fields. In the case of such an optical waveguide produced on a substrate such as, e.g., silicon or silica, it is constituted of a core layer which transmits light and cladding layers which have a lower refractive index than the core layer. This optical waveguide, illustratively stated, has a three-layer structure comprising an undercladding layer, a core layer formed thereon, and an overcladding layer which surrounds the core layer. Various polymeric materials are used as materials for forming the two cladding layers, i.e., the undercladding layer and the overcladding layer, and as materials for forming the core layer.
For example, fluorinated polyimides, which are satisfactory in light transmission as well as heat resistance, have been used as a material for forming the two cladding layers and a material for forming the core layer. However, the fluorinated polyimides have poor adhesion to substrates. It has hence been proposed to form an undercoat layer between a substrate and a cladding layer from the standpoint of improving adhesion between the substrate and the cladding layer. It has also been proposed to incorporate a coupling agent into the undercoat layer in order to further improve adhesion (see, for example, patent document 1).
Since the layer constitution described above necessitates complicated production steps, a technique is being investigated in which a coupling agent is directly incorporated into a cladding layer to secure adhesion between a substrate and the cladding layer without forming an undercoat layer.
Patent Document 1: JP 2002-90559 A
However, formation of an undercladding layer from the material for undercladding layer formation which contains a coupling agent has the following drawback. When a core layer having a predetermined pattern is formed on the undercladding layer through patterning by a wet process using an alkaline developing solution or the like, the undercladding layer opacities. This opacification of the undercladding layer is by no means desirable for optical waveguides because it results in a propagation loss.